The proposed research focuses on the molecular analysis of gene expression in the viruses causing squash leaf curl disease (SqLCV- NR and SqLCV-BR) and maize streak disease (MSV). These studies have as their primary goal the functional mapping of the genomes of SqLCV and MSV in order to: (1) identify viral genomic elements important in host range and pathogeneses; (2) define the elements required for gene expression, specifically replication and transcription, in these viruses in an effort to understand regulation in geminiviruses and as a model for gene expression in monocots and dicots; and, (3) begin to identify cell factors important in gene regulation in plants. Biological assays for the expression of agroinoculation, sequence elements of the closely related SqLCV-NR nd SqLCV-BR, which have different host ranges, will be exchanged and mutants studied for their host range phenotypes and ability to cause systemic disease. MSV deletion and gene substitution mutants will also be analyzed for their ability to infect monocots or dicots. These will be investigated through mutant analysis and as such sequence elements are identified they will be used in binding assay to identify cell and viral factors involved in gene regulation. In addition to pathogens, these studies will add to our understanding of basic mechanisms of host range restriction, pathogenesis and eukaryotic gene expression.